A secondary battery having a high energy density is needed as the markets of notebook computer, mobile phone, electric vehicle and the like are rapidly expanded. As a method for obtaining a secondary battery having a high energy density, there has been known a method of using a high capacity negative electrode material, a method of using a nonaqueous electrolytic solution having good stability, or the like.
Silicon oxides or silicates used as negative electrode materials for a secondary battery are disclosed in Patent Document 1. A negative electrode used for a secondary battery provided with an active material layer comprising carbonaceous particles capable of absorbing and emitting lithium ions, metal particles capable of forming an alloy with lithium and oxide particles capable of absorbing and emitting lithium ions is disclosed in Patent Document 2. A negative electrode material used for a secondary battery is disclosed in Patent Document 3 in which the surfaces of particles having a structure with silicon microcrystals dispersed in a silicon compound are coated with carbon.
However, when silicon oxides or complexes of silicon and silicon oxides are used as a negative electrode, Li—Si—O compounds are formed during an initial charge process, so that some of Li from a positive electrode cannot be used in a reversible charge/discharge reaction. Consequently, even though a material having greater capacity than carbon is used for the negative electrode, it is difficult to obtain a secondary battery having a high capacity.
To compensate such an irreversible capacity, a method of pre-adding lithium to a negative electrode material has been known. Patent Document 4 discloses a nonaqueous electrolytic solution secondary battery having lithium added previously by attaching lithium to a portion of negative electrode that is not faced with a positive electrode. Patent Document 5 discloses a nonaqueous secondary battery in which a negative electrode contains a material formed by pre-doping lithium on SiOx (0.3≦x≦1.6). As such, an irreversible capacity is generated due to an irreversible reaction of a negative electrode active material with lithium during an initial charge process and hence lithium absorption. To solve such a problem, by pre-supplementing lithium by an amount corresponding to the irreversible capacity, a reduction in battery capacity after initial charge/discharge may be suppressed, and an energy density of secondary battery may be increased.
Further, as an attempt for compensating such an irreversible capacity of a negative electrode without altering a current battery manufacturing process, a method of adding a Li-rich compound to a positive electrode has been known. Patent Document 6 discloses an electrode comprising a first electrode material having Li2NiO2 and a second electrode material having LiTMO2 (TM: a transition metal element selected from Ni, Co and Mn). Patent Document 7 discloses a positive electrode active material used for a lithium secondary battery containing a lithium nickel oxide in which some of nickel in Li2NiO2 is substituted by other elements. Patent document 8 discloses a positive electrode used for a lithium secondary battery containing Li2CuO2.